In a process of manufacturing a semiconductor device, a plasma process, such as dry etching or ashing, is performed on a semiconductor wafer by using a plasma generated from a process gas. In a plasma processing apparatus performing the plasma process, for example, a pair of parallel electrodes, each having a plate shape, are positioned in parallel one above the other. High frequency power is applied between the pair of parallel plates to generate a plasma from a process gas. When the plasma process is carried out, the wafer is mounted on a lower electrode serving as a mounting table.
With an increasing demand for a plasma which has low ion energy and high electron density, high frequency power applied between the electrodes tends to have a very high frequency, e.g., 100 MHz, compared to conventional frequencies of, e.g., less than 20 MHz. However, it has been observed that if the frequency of the applied high frequency power rises, an electric field is strengthened at a space above the center of the surface of the electrode, i.e. the center of the wafer, but is weakened at a space above a peripheral portion of the surface of the electrode. Such non-uniform distribution in electric field may cause a non-uniform electron density in the plasma, so that, e.g., etching rate may vary depending on a position within the wafer in dry etching using ions. Thus, a problem may occur in that satisfactory in-plane uniformity can not be obtained in dry etching.
In order to cope with this problem, there is disclosed a plasma processing apparatus, which can make the electric field strength distributed uniformly and improve in-plane uniformity in a plasma process by embedding a dielectric layer made of, e.g. a ceramic, at the central of a top surface region of the lower electrode, i.e., mounting table (see, e.g., Japanese Patent Laid-open Application No. 2004-363552 and corresponding U.S. Patent Application Publication No. 2005/0276928 A1)
As shown in FIG. 10A, when high frequency power is applied from a high frequency power supply 82 to a lower electrode 81 in a plasma processing apparatus 80, a high frequency current flows along a surface of the lower electrode 81 to an upper part thereof by the skin effect, and then flows through a wafer W toward the central portion thereof. At this time, a part of the current leaks from the central portion of the wafer to the lower electrode 81 and then flows outward inside the lower electrode 81. Here, the high frequency current may more deeply penetrate into the portion of the lower electrode 81 at which a dielectric layer 83 is embedded than the other portions of the lower electrode 81; and accordingly, a hollow cylindrical resonance of TM mode is generated at the central portion of the lower electrode 81. Consequently, the electric field strength can be lowered at a space above the central portion of the wafer W, to thereby make the electric field strength uniformly distributed at the space above the wafer W.
Since a plasma process is normally conducted under a depressurized atmosphere, an electrostatic chuck 84 is used to firmly mount the wafer W in the plasma processing apparatus 80 as shown in FIG. 10B. A conductive electrode film 85 is interposed between a lower member and an upper member, which are made of a dielectric material, e.g., alumina, in the electrostatic chuck 84. During a plasma processing, high voltage DC power is supplied from a high voltage DC power supply 86 to the electrode film 85 to generate a coulomb force on a surface of the upper member of the electrostatic chuck 84, whereby the wafer W is electrostatically adsorbed and fixed.
Each component of the plasma processing apparatus 80 can be treated as a component of an electric circuit for a high frequency current. Further, the wafer W is formed of a semiconductor such as silicon, and thus, the wafer W is also considered as a component of the electric circuit. Since the wafer W is mounted in parallel with the electrode film 85 when the wafer W is electrostatically attracted to the electrostatic chuck 84, the wafer W and the electrode film 85 are considered to serve as resistors arranged in parallel in the electric circuit.
As a consequence, the value of a high frequency current flowing through the wafer W is dependent on a resistance of the wafer W and a resistance of the electrode film 85. For instance, when the resistance of the electrode film 85 is larger than that of the wafer W, high frequency current mainly flows from a peripheral portion to the central portion of the wafer W (see FIG. 11A). In such a case, a large potential difference occurs between the peripheral portion of the wafer W and the central portion of the wafer W as shown in FIG. 11B, so that a gate oxide film 87 is charged up and deteriorated.
Further, when the resistance of the electrode film 85 is very small, the high frequency current leaking from the central portion of the wafer W to the lower electrode 81 side can readily flow through the electrode film 85, and thus the high frequency current can not penetrate deep into the central portion. As a consequence, a hollow cylindrical resonance of TM mode is not produced and the electric field strength is non-uniformly distributed, which causes electron density of a plasma to be increased in a space above the central portion of the wafer W. Accordingly, a DC-like current flows between the central portion and the peripheral portion of the wafer W. This case also has the same problem as that described above in that the gate oxide film 87 of semiconductor devices disposed on the wafer W is charged up and deteriorated.